This is plaguing me. I've created a super-simple scene to illustrate: just a particle emitter with the constant shader attached, and a camera.


Rendered with just depth of field enabled:



Rendered with just motion blur:



But with both, everything goes splotchy:



Doesn't seem to be related to Pixel Samples, or Geo Time Samples. How do I cure it?

Nope - my bad. Pixel Samples does cure it, though you need to whack it right up (32x32 seems good):




Leaving this post here for future reference; may help someone else. Never realised I'd need so many Pixel samples.

Try adding the render property vm_decorrelatedof “Decorrelate Depth of Field from Motion Blur Samples”

It might require even more samples to be smooth, but should help eliminate correlation artifacts (odd boxy noise/missing areas)

Dof requires many times as many samples as a fully opaque render, because of the probabilistic nature of raytracing. When a small point is blurred out into a large disc, the probability of a ray hitting a given particle is reduced by the ratio of the areas. So a particle that when blurred is 10x larger is going to take 100x as many samples to resolve.

Thank you! That's sorted it! Even at 5x5 Pixel samples, as per previous images, that's sooooo much better.

I can up the sampling a bit more, but even with this level of noise, my de-noising plugin can tidy it up - something it couldn't do with the blotchy artifacts from before.

Yay

So - I wonder - under what circumstances does the correlated samples thingy actually benefit you?